The present invention relates to a method for monitoring and regulating composition and layer thickness of metallically conductive alloy layers during their manufacture by means of electrical resistance measurement, whereby the individual alloying components are cyclically deposited layer-wise in chronological succession.
The exact knowledge of the composition and the thickness of layers which are produced by means of vapor-deposition or sputtering, for example by means of simultaneous sputtering of two or more targets ("co-sputtering"), is of great significance for the fabrication of thin film circuits or integrated circuits.
Whereas the composition has an extremely sensitive influence on the electrical and metallurgic properties (for example, diffusion, reaction, etc.), knowledge of the thickness is also essential given layers which are employed, for example, as resistance or etching stops. In the course of continuing miniaturization in semiconductor technology, the tolerances with respect to composition and thickness have become very tight, so that a measurement and control of these two values is already indispensable during the layer production in order to obtain a sufficiently high yield.
Given chrome-nickel thin film resistors, the temperature coefficient of the electrical resistance, which must lie, for example, in the range from -25.degree. through +25.degree..times.10.sup.-6 .degree./K. is very sensitively influenced by the composition. In order to be able to reproducibly observe these values, the concentration may only fluctuate by .+-.3% atomic number part. Since, moreover, a high stability is also demanded of this material which in turn depends on the quantity of oxygen incorporated into the layer, the oxygen concentration must also be controlled.
Finally, a resistance structure with a narrow tolerance value of resistance is generated from the layer which is deposited in surface-wide fashion, and which is generated by means of a photo etching process. The fluctuations in layer thickness must therefore lie below .+-.3%.
Given tantalum-silicon layers employed in VLSI technology as low-resistance gate material or as interconnects, the specific electrical resistance is highly influenced by the composition. Furthermore, only layers having a specific concentration have the property of forming a protective SiO.sub.2 layer in an oxidizing atmosphere at temperatures around 900.degree. C. In this case, too, a reproducible oxide growth is only obtained when the composition fluctuates only slightly (.+-.2% atomic number part) around a fixed value.
In order to obtain homogeneous layers with respect to layer thickness and composition, it is known to move the substrates provided for the coating during coating (for example, rotation, planetary movement). Since modern coating systems frequently comprise a lock via which the substrates are introduced into the actual coating chamber, the substrate is not stationary. This greatly complicates an in-place identification of the layer thickness or of the composition. One possibility of determining the layer thickness is to measure the resistance at a stationary substrate which is provided with electrical contacts and is coated at the same time as the other, moved substrates. Reference is made in this regard to an article by S. C. P. Lim and D. Ridley in Solid State Technology, February 1983, pages 99 through 103 and to an article by I. A. Turner et al in J. Sci. Instrum., Vol. 40, pages 557 through 561, both incorporated herein by reference.
A further method consists of installing one or more stationary crystal oscillators in the system, the resonant frequency of said oscillators changing in a defined fashion with increasing weight per unit area due to a vapor-deposited or sputtered material.
The disadvantage of these methods is that a calibration must be carried out due to the different topical positions between substrate and oscillator or reference substrate, respectively.
When, for example, one wishes to control the composition of a binary alloy layer with the assistance of crystal oscillators, then it must be guaranteed that at least two oscillatory crystals are mounted in the system, whereby each oscillator is coated by only one source. Conclusions can be drawn regarding the composition of the layer from the ratio of the momentary weight per unit area.
A further, very exact possibility of regulating the layer composition is to utilize a mass spectrometer, given which the material deriving from the two vaporizer sources proceeds into the ion source and is registered. Here, too, there is the disadvantage that the ion source is stationary and that an oscillatory crystal is required for the identification of the layer thickness. Furthermore, this method cannot be applied given sputtering systems, or is too imprecise since the deposition occurs at an argon pressure of about 10.sup.-2 mbar.